Pterosaurs were likely floaters: evidence from manus only tracks

Yesterday we reviewed Hone and Henderson (2013) who conducted computational experiments with four misbegotten digital pterosaur models and reported that pterosaurs were unlikely floaters that would have struggled to keep their noses above the surface and so risked drowning, despite their air-filled skeletons.

Unfortunately
the Hone and Henderson results don’t agree with the facts as told by manus-only tracks, that can only be made by floating pterosaurs. As Hone has done in previous papers, these are all conveniently omitted. Case in point: the Summerville tracks (Lockley et al. 1996, Fig. 1).

Summerville (Late Jurassic) manus only tracks (Fig 1), likely made by a sister to Jidapterus, a protoazhdarchid with rather big fingers.

Is this the only explanation?
Oh, sure some have said that pterosaurs pressed their hands more deeply into the matrix and footprints were thereafter erased by geological processes. But doesn’t this strike you as trying to make excuses, on the order of Elgin, Hone and Frey’s infamous “membrane shrinkage“?

The large Las Hoyas track
is impossible to fit to a pterosaur manus. No pterosaur has a longer and more robust manual digit 2 than 3. Some have these two digits subequal in length, but to scale these up to the track size creates a truly gigantic pterosaur. Vullo et al. 2009 got it right when they decided it belonged to a theropod dinosaur foot.

The “first Gondwana pterosaur tracks” (Calvo and Lockley 2001) can all be matched to Tapejara-like (Fig. 4, 5) trackmakers. The Candeleros Member of the Rio Limay Formation (Albian–Cenomanian) at Lake Ezequiel Ramos Mexía, Neuquén Province, Argentina is contemporary with Tapejara bones on the east coast of Brazil. The palaeoenvironmental setting of the track beds was a lake shoreline, where dinosaur tracks also occur.

Above, manus only tracks (Calvo and Lockley 2001) matched to Tapejara.

Moganopterus, a cycnorhamphid, is a good model for the trackmaker of the Maastrichtian Price (Utah) racks, merely with a shorter digit 2 than Cycnorhamphus (Fig. 5). Unfortunately Moganopterus is from the Early Cretaceous of China.

If you’re interested
in finding a better match for any of these tracks, you are welcome to try. I had a catalog of pterosaur manus and pedes at reptileevolution.com and a matrix of pterosaur traits that made my search go rather quickly.

References
Calvo JO and Lockley MG 2001. The first pterosaur tracks from Gondwana. Cretaceous Research 22:585-590.
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica 56 (1), 2011: 99-111. doi: 10.4202/app.2009.0145
Hone DWE, Henderson DM 2013. The posture of floating pterosaurs: Ecological implications for inhabiting marine and freshwater habitats, Palaeogeography, Palaeoclimatology, Palaeoecology (2013 accepted manuscript), doi: 10.1016/j.palaeo.2013.11.022
Lockley MG, Logue TJ, Moratalla JJ, Hunt AP, Schultz RJ and Robinson JW 1995.  The fossil trackway Pteraichnus is pterosaurian, not crocodilian: implications for the global distribution of pterosaur tracks. Ichnos, 4: 7–20.
Lockley MG, Hunt AP and Lucas SG 1996. Vertebrate track assemblages from the Jurassic Summerville Formation and correlative deposits. – In: Morales M. (Ed.), The Continental Jurassic. Museum of Northern Arizona Bulletin, 60: 249–254.
Lockley MG and Wright JL 2003. Pterosaur swim tracks and other ichnological evidence of behavior and ecology. – In: Buffetaut E and Mazin JM (Eds), Evolution and Paleobiology of Pterosaurs; Geological Society, London, Special Publications 217:297-313.
Lockley M, Harris JD and Mitchell L 2008. A global overview of pterosaur ichnology: tracksite distribution in space and time. Zitteliana B28: 185-198.pdf
Mickelson DL, Lockley MG, Bishop J, Kirkland J 2004. A New Pterosaur Tracksite from the Jurassic Summerville Formation, Near Ferron, Utah. Ichnos, 11:125–142, 2004
Parker L and Balsley J 1989. Coal mines as localities for studying trace fossils. In: Gillette DD and Lockley MG (Eds), Dinosaur Tracks and Traces; Cambridge (Cambridge University Press), 353–359.
Pascual Arribas C and Sanz Perez E 2000. Huellas de pterosaurios en el groupo Oncala (Soria España). Pteraichnus palaciei-saenzi, nov. ichnosp.  Estudios Geologicos, 56: 73–100.
Vullo R, Buscalioni A D, Marugán-Lobón J and Moratalla JJ 2009. First pterosaur remains from the Early Cretaceous Lagerstätte of Las Hoyas, Spain: palaeoecological significance. Geological Magazine, 146: 931-936.